The present invention relates to sensors for continuously measuring the pH of absolution, such as in chemical processing.
A wide variety of types of sensors have been developed to measure the pH of chemical solutions in industrial processes. One type of pH sensor employs a glass electrode in the form of a hollow glass tube containing a solution of a neutral pH (i.e. pH=7.0). A tip of the glass tube that is exposed to the solutions being tested is sensitive to hydrogen ion activity and pH difference across the inner and outer surfaces. An electric wire extends through the wall of the glass tube into the neutral pH solution. The glass electrode is spaced from a metal electrode and. the electrodes are exposed to the solution that has an unknown pH.
The measurement process is predicated on the principle that a hydrated gel layer forms between the outer surface of the glass and the solution being tested. Depending upon the pH of that solution being tested hydrogen ions migrate into or out of the gel layer. For example in an alkaline solution, hydrogen ions migrate out of the gel layer developing a negative charge in that layer. Because the solution inside the glass tube has a constant, neutral pH, an electric potential develops across the glass membrane due to the difference in the inner and outer electric charges. This produces a first electrical signal between the glass and metal electrodes which varies with the pH of the solution.
A second glass electrode is placed within another chemical solution that has a known pH and thus producing a second electrical signal. The unknown pH of the solution being tested can be determined by comparing the two electrical signals.
The pH sensors are commonly used in industrial environments that have significant ambient electrical noise. Depending upon the routing of the conductors between the electrodes and preamplifiers in the processing circuits, the electrical noise can be induced into the electrode signals and applied to the preamplifiers. Even when the preamplifiers are incorporated into the housing for the electrodes, electrical noise can be induced into he conductors that connect the remote sensor to the apparatus that derives the pH measurement, thus adversely effecting accuracy of that measurement.
Therefore it is desirable to provide a sensor which accurately measures pH of the chemical solution under test, and has a relatively high degree of immunity to ambient electrical noise.
The present invention provides a pH sensor that has a compact housing that contains sensing electrodes and signal preamplifiers. The sensor is designed to limit the effects that ambient electrical noise has of the signals being produced.
The sensor includes a housing with a section having an exterior surface for exposure to the solution being tested. A measuring glass electrode and a circuit ground electrode extend through the exterior surface of the housing section. First and second preamplifiers are mounted within the housing and a first electrical conductor connects the measuring glass electrode to the first preamplifier. A second electrical conductor connects the reference glass electrode to the second preamplifier. The first and second electrical conductors have substantially the same length so that ambient electrical noise equally affects the different signals that the conductors carry.
Another aspect of the invention provides a ferrite sleeve through which passes the electrical cable that carries the output signals from the first and second preamplifiers to remote signal processing circuits.